1. Technical Field
This present invention relates to a display device that uses electroluminescent elements (hereinafter, referred to simply as EL).
2. Background Art
In recent years, among many kinds of flat panel display devices, those display devices that use electroluminescent elements have drawn public expectations. The display device using EL elements has advantages, such as a spontaneous light-emitting property, superior visibility, a wide viewing angle and a high response speed. Moreover, EL elements that have been currently developed include inorganic EL elements in which an inorganic material is used for its illuminant, and organic EL elements in which an organic material is used for its illuminant.
The inorganic EL element using an inorganic phosphor, such as zinc sulfide as an illuminant, has such a structure that electrons, accelerated by an electric field as high as 106V/cm, are made to collide with the luminescence centers of the phosphors so as to be exited and when they are alleviated, light is emitted. The inorganic EL elements include dispersion-type EL elements having a structure in which phosphor powder is dispersed in a polymer organic material or the like with electrodes being formed on the upper and lower sides thereof, and thin-film-type EL elements having a structure in which two dielectric layers are formed between a pair of electrodes with a thin-film phosphor layer being sandwiched between the dielectric layers. Although the dispersion-type EL elements can be easily produced, they have low luminance and short service life, with the result that the application thereof is limited. In contrast, with respect to the thin-film-type EL elements, those elements having a double insulating structure, which have been proposed by Inokuchi et al., in 1974, exhibit high luminance and long service life, and have been put into practical use as displays for use in vehicles, as shown in Japanese Patent Laid-open Publication No. 52-33491.
Referring to FIG. 64, the following description will discuss the conventional inorganic EL element. FIG. 64 is a cross-sectional view obtained when an EL element 50 using a thin-film dielectric member 55 is viewed in a direction perpendicular to the light-emitting face thereof. The EL element 50 has a structure in which a transparent electrode 52, a thin-film dielectric layer 53, a phosphor layer 54, a thick-film dielectric layer 55 and a back electrode 56 are stacked on a substrate 51 in this order. A light emission from the phosphor layer 54 is taken out from the transparent electrode 52 side. The thick-film dielectric layer 55 has a function for regulating an electric current flowing through the phosphor layer 54, can suppress a dielectric breakdown in the EL element 50, and also functions so as to provide a stable light-emitting characteristic.
Moreover, upon configuring a display device by arranging a plurality of EL elements two-dimensionally, a plurality of EL elements aligned over the same row may be made to use the common transparent electrode, and a plurality of EL elements aligned over the same column may be made to use the common back electrode. In this case, one transparent electrode serves as a data electrode that extends in a column direction, and one back electrode serves as a scanning electrode that extends in a row direction so that a plurality of data electrodes that are in parallel with each other and a plurality of scanning electrodes are patterned into stripes that are made orthogonal to each other. By applying a voltage to a specific pixel selected within the matrix of the data electrodes and the scanning electrodes, a display device of a passive matrix driving system, which carries out a desired pattern display, can be obtained.
In this case, however, when the display device using the inorganic EL elements is utilized as a high-quality display device, such as a television, luminance of about 300 cd/m2 or more is required, with the result that the device becomes insufficient from the viewpoint of light emission luminance. Moreover, with a display device of a passive matrix driving system, when the number of the scanning lines increases along with the developments of a high definition system, the luminance is further lowered. Furthermore, in order to drive the above-mentioned inorganic EL element, normally, an AC voltage of about 200V needs to be applied with a high frequency of several kHz, with the result that problems arise in which an active element such as a thin-film transistor is not applicable and in which a high-cost driving circuit is required; therefore, there are still some problems in order to put this system into practical use.
As a result of extensive studies made by the inventors of the present invention to achieve a low voltage and high luminance of the inorganic EL element, the inventors have found an inorganic EL element that can be driven by using a direct current, and emits light with high luminance by using a low voltage of several 10V that is sufficiently low in comparison with the voltage required for the conventional inorganic EL element (hereinafter, referred to as “direct-current driving type inorganic EL element”).